export type MaybePromise<T> = T | Promise<T>;
export type AuthTokenResolver = () => MaybePromise<string | null | undefined>;
export type HeaderResolver = () => MaybePromise<HeadersInit | undefined>;
export declare function mergeHeaders(...sources: Array<HeadersInit | undefined>): Headers;
export declare function resolveAuthToken(authToken: string | null | undefined, getAuthToken?: AuthTokenResolver): Promise<string | null>;
export declare function resolveHeaders(headers: HeadersInit | undefined, getHeaders?: HeaderResolver): Promise<Headers>;
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